Tried using a torch and chisel tonight, no luck. Even had the whole system from the muffler back free from the hangers to get as much movement as possible but it just won't budge. I'm going to try a dead blow and a wood block next week, after heating up the sleeve again...

I'm trading my Corsa Touring for a Corsa Sport, but I'm having a hell of a time getting my Touring muffler off. Clamps are loose, but that muffler is not sliding at all on the exhaust pipes... I'm going to grab a torch tomorrow and heat up the larger pipes on each end, but are there any other tricks I can try to get it off? I'm working on my driveway with the truck on the ground, so my leverage is limited. It seems like I could try removing muffler back from the hangers, but even the hangers seem like a PITA to remove. Any ideas? Muffler only has 2500 miles on it, so there is no rust holding it together.

Got my switch installed today in my 2018 Sierra All Terrain X. Trigger isn't going anywhere yet, but I have to agree with lapoolboy, the off-road button is very sporatic. Sometimes you have to mess with the fogs before the off-road will turn on/off, mostly in park though. While driving, I didn't try it much but I believe it would reliably work only when the fogs were on. Gonna play with it a bit, see if I can nail down what it wants to happen to work.

The sport bar will fit fine, the light package linked above might not work with your year. I believe the kit requires a BCM update, which may not be available for 2015's because the ATX wasn't available then.
Looks like you found out your tonneau cover won't fit, but this one will:
https://accessories.gmc.com/product/2017/GMC/Sierra 1500/short-box-premier-roll-up-tonneau-cover-by-advantage®---associated-accessories-19355362?categoryId=98027
I have that on my 2018 ATX (standard bed though) and it fits fine. Gets a little tight when rolling past the bars, but can roll all the way up.

I have the GM cat-back exhaust that comes on the All Terrain X models, which I believe is a Corsa Touring muffler. I might be interested in trading, so long as everything fits right up. Let me know. I'm in NY, so we would have to ship.

I did this a couple weeks ago, with a composite shim stuck behind there. Definite improvement with throttle response.
As a mechanical engineer, I tried to wrap my head around why they designed it the way they did. My best guess is to dampen vibrations. With the shim, I can definitely feel more vibration in the go pedal compared to floating from the factory. I might try replacing the shim with a dampener of some sort, see if I can get some of the road feedback down.

I also got my switch, going to put it in the same time I get the wiring from Phil...
Lights came today, I like them but not sure it's exactly what I'm looking for. The mounting plate bends a little on the sport bar, so the lights sit at a bit of an angle. Might be able to shim them straight, or move them more towards the center. Or I might use these as cargo/reverse lights and hang them under the bar.

https://www.amazon.com/gp/aw/d/B00T9GC59U
A couple of cheap 12" light bars to go on my All Terrain X sport bar. The wiring should already be run to the front, so I'll tap into that and run a relay from the trigger on this switch. Then wire the lights up to the factory harness in the bar. That's the plan, hope it's that easy...
If these lights don't last, I'm looking to upgrade to Rigid Industries 10" bars. I just couldn't justify the price first go around, since the lights will be used sparingly as it is.
Now to find a relay.

I just want to confirm, if I wire this switch right up like the video, the light will come on whenever the switch is long pressed? It isn't limited to high beams only, like I think the stock harness is (the only that comes with the 2 ATX lights)?

Interested in this as well...
Also, I was thinking about mounting some lights on the side where it says Sierra, maybe an 18" long light bar with red covers. Tap into my puddle lights (which come on with my interior lights thanks to the diode jump) so it lights up when I unlock and lock my truck.

I should clarify, I have the GM catback as well and am also looking to trade for the sport. I have the 5.3L double cab, but I don't think that matters for just swapping the mufflers. I'm assuming touring and sport mufflers are the same size...
Located in NY.